Although the use of colored glass is not especially limited, it can be applied for privacy glass or the sun roof of car safety glass and building glass, etc. As compared with general soda-lime glass, colored glass can lower the visibility in motor vehicles because of its low visible light transmittance (LTA) and can reduce heat absorption into motor vehicles because of its low solar energy transmittance (Te). In addition, since it has a low ultraviolet rays transmittance (TUv), damage to fabrics and discoloration or decomposition of articles inside motor vehicles due to ultraviolet rays can be prevented.
Various elements can be incorporated into colored glass in order to improve the glass color, UV-blocking effect, and solar energy absorption effect. The representative elements used in colored glass include iron (Fe), cobalt (Co) and selenium (Se), and manganese (Mn), nickel (Ni), copper (Cu), chrome (Cr), titanium (Ti), cerium (Ce), etc. may be further used. Each of these elements has a unique coloring effect, and absorption characteristics of ultraviolet rays and solar energy. Such characteristics result from the phenomenon that each element absorbs its specific wavelength. Accordingly, the desired color and transmittance may be designed through the combination of added elements in appropriate ratios.
By properly adjusting various elements as explained above, it is possible to prepare dark neutral gray, neutral gray, or dark neutral green-gray glass with low color purity, having the aesthetic merit of harmonizing with buildings and motor vehicles, and the function of absorbing ultraviolet rays and solar rays. Various compositions of such gray soda-lime glass have been known and can be divided into two groups, in which one group is prepared by using Fe, Co, and Se as basic elements for coloring while the other group further uses additional elements.
U.S. Pat. No. 4,873,206 discloses a glass composition comprising 0.6 to 1.0% by weight of total Fe2O3, 0.01 to 0.02% by weight of CoO and 0.005 to 0.02% by weight of Se, and not containing Ni and Cr. However, this composition has a visible light transmittance (LTA) of about 25 to 30% as measured for a reference thickness of 4 mm and thus is not suitable for providing the function of privacy protection which is an important function of gray glass applied for sunroof or rear privacy glass.
Korean Patent No. 10-0227250 discloses a glass composition comprising 1.4 to 4% by weight of total Fe2O3, 0.02 to 0.005% by weight of CoO, 0.005% by weight or less of Se and 0.24% by weight or less of CoO+Se+Cr2O3. However, since the above composition has a high content of total Fe2O3, Fe2+ is inevitably generated and this causes problems such as heat conductivity decrease, fuel cost increase, temperature decrease in the bottom of a melting furnace, melting quality degradation, etc. In addition, Cr used in this composition is known as a poorly soluble material and as its amount increases, non-molten materials are generated and non-uniform color is caused thereby. Cr is known as a representative green colorant, and thus a dark green color is provided by using a very small amount thereof. Accordingly, if this element is used, there is a difficulty in production of glass with high color uniformity. Furthermore, the use of excessive Fe2O3, CoO and Se increases color purity, and thus it is not suitable for harmonizing with various colors of buildings and interiors.
Korean Patent No. 10-0295379 discloses a glass composition comprising 1.3 to 2% by weight of total Fe2O3, 0.02 to 0.04% by weight of CoO, 0.0002 to 0.003% by weight of Se, and 0.01 to 0.05% by weight of Ni. However, in case of containing Ni like this, it may exist as NiS during the glass-melting procedure and NiS is known to cause glass breakage by volume expansion after the glass-tempering procedure. Concretely, dark, neutral gray-colored, low-transmittance glass is mainly used for motor vehicles and since a glass for motor vehicles must be tempered, the use of Ni increases the possibility of glass breakage. That is, Ni is an element which is required to be limited in its use in glass compositions for motor vehicles.